1. Technical Field
The present invention relates to electro-optical devices and electronic apparatuses suitable for display devices that control their display luminances.
2. Related Art
A liquid crystal panel includes two substrates, such as glass substrates or quartz substrates, and liquid crystal filled therebetween. The substrates are provided with electrodes, and image signals are supplied to the electrodes. The optical characteristic of the liquid crystal between the electrodes of the substrates changes according to the image signals. That is to say, by applying voltages based on the image signals to the liquid crystal between the electrodes of the substrates, the alignment of liquid crystal molecules is changed. Thus, the light transmittance in each pixel changes according to the image signals, and an image display according to the image signals is performed.
In order to perform a high-luminance display in such a liquid crystal panel, in general, a backlight unit is provided at the back of a liquid crystal panel. A backlight unit that improves its illumination uniformity using a light guide plate has been developed. By illuminating a display region of a liquid crystal panel with a backlight unit, a display on the display region can be observed with a sufficient luminance.
The viewability of display of a liquid crystal panel changes depending on the ambient brightness. For example, when the ambient light is bright, the display is less viewable, and therefore the illumination of the display region needs to be brighter. Conversely, when the ambient light is sufficiently dark, the display region need not be brighter than necessary.
JP-A-2003-78838 discloses a technique to provide an easily viewable display independently of the ambient brightness. In the technique, the ambient light is detected, and the luminance of a backlight unit is controlled on the basis of the feedback information.
The apparatus of JP-A-2003-78838 uses a discrete component as a light sensor for detecting the ambient light. Therefore, the light sensor needs to be mounted on a flexible printed board. This increases the man-hour and the cost.
To solve this problem, a PIN diode light sensor can be formed on a substrate constituting a display panel, such as a liquid crystal panel. In this case, the PIN diode is formed in the same layer as the semiconductor layer in the display region.
The semiconductor layer in the display region is set to a comparatively small thickness in order to obtain desired transistor characteristics. However, in the case where the semiconductor layer in the PIN diode forming region is thin, incident light passes through the semiconductor layer, and photogenerated charges cannot be efficiently generated. That is to say, light is not efficiently incident on the intrinsic layer, and a sufficient light sensitivity cannot be obtained.